The most widely used water purification method is distillation, which is accomplished by adding heat to a solution to generate pure water vapor. The water vapor is then usually condensed to produce pure liquid water. The amount of heat required to evaporate the water is about 1000 BTU per pound. To condense the vapor, an additional 1000 BTU per pound must be removed from the vapor. Ideally, one can cascade the evaporation and condensation processes to reduce energy input requirements to the 1000 BTU per pound required for the initial evaporation. In reality, much more energy is required than in the ideal case. State of the art cascading distillation systems require at least 50 BTU per pound of product. Systems with such capability are large, complex, and expensive.
Another purification technique, freezing purification, relies on the phenomenon that when a water solution freezes, it will reject the impurities (both solids and gases) contained therein. Thus, the frozen portion of a partially frozen water solution will have far lower impurity concentration than will the unfrozen liquid portion.
It has been suggested that freezing techniques be employed to purify large quantities of saline water, for example, to satisfy municipal demand. For example, U.S. Pat. No .3,404,536, issued Oct. 8, 1968 to Aronson, discloses a dual chamber, flash freezing purification system. The Aronson system, designed to process up to approximately 250,000 gallons of saline water per day, is very complex. Aronson sprays 37 degree Fahrenheit saline water from spray nozzles in a first low pressure chamber, so that water will flash freeze on screens located adjacent the spray nozzles. The chamber is kept at low pressure during the freezing process by 24 degree Fahrenheit refrigerant flowing in coils that run through the first chamber. At a later time, the ice-covered screens are washed, and saline liquid is drained away. Still later, 39 degree Fahrenheit refrigerant is caused to flow through the coils in the first chamber, so as to evaporate liquid in a reservoir in the first chamber, which in turn causes the ice on the screens to melt. To complete the cycle, the melted ice is drained away.
While 24 degree Fahrenheit fluid flows through the coils in the first chamber, 39 degree Fahrenheit fluid is caused to flow through the coils of an identical second chamber (also having spray nozzles from which saline water is sprayed onto screens), to melt ice on the screens in the second chamber. When the freezing operation is concluded in the first chamber, and valves are switched to cause 39 degree fluid to flow through the first chamber, valves are simultaneously switched to cause 24 degree refrigerant to flow through the coils of the second chamber. Thus freezing and melting operations are performed in the first and second chambers 180 degrees out of phase with respect to each other.
The Aronson system requires a very complex heat pump subsystem, including valves for controlling the flow of refrigerant to both chambers. The heat pump periodically reverses the flow of refrigerant to the chambers, so that 39 degree refrigerant flows into the heat pump to one of the chambers in one tube, and 24 degree refrigerant later flows out of the heat pump to the same chamber and in the same tube.
In another embodiment, Aronson uses an absorption/desorption refrigerant (such as lithium bromide) together with alternating streams of 85 degree Fahrenheit water and 95 degree Fahrenheit steam in the coils of the refrigeration subsystem. In all embodments, a large apparatus including a complicated system of valves and a complex heat pump are required. Due to the high heat transfer requirements for processing large volumes of saline water using flash freezing, Aronson system'srefrigeration components must accordingly have complex design. Where lithium bromide is used, a refrigerant leak in the Aronson system risks contamination of the water being processed.
Another conventional freezing purification technique is disclosed in U.S. Pat. No. 3,212,272, issued Oct. 19, 1965 to Sommers, Jr. The Sommers system employs stacks of thermoelectric heat pumps positioned between adjacent compartments. Saline water fills alternating ones of the compartments. The pumps are electrically connected to a DC power source so that their "cold" junctions (the heat absorbing junctions) face the liquid saline water. The pumps are selectively activated (the lowermost pump first and the uppermost pump last) so as to freeze the liquid water in the compartments from the bottom up. The "hot" junctions of the pumps face ice contained the other alternating ones of the compartments, so as to melt the ice from the bottom up as the pumps are selectively activated. The melted ice drains to a fresh water storage area. Then, the electrical connections of the pumps are reversed and the empty compartments (which contained ice that has melted away) are filled with saline water. The pumps are again selectively activated from the bottom up, so as to freeze and melt alternate ones of the compartments.
The compartments of the Sommers system are inclined to permit liquid to drain off the inclined top surface of the ice contained therein. This configuration will result in a nonuniform rate of ice formation, due to inherent nonuniformities in salt concentration and specific gravity of the fluid in the compartments, unless this tendency is counteracted by supplemental control means. An additional disadvantage of the Sommers system is that the thermoelectric pumps used are energy inefficient, and accordingly uneconomical to operate. For example, the thermoelectric heat pump Model CP 5-31-06L (manufactured by Melcor) has a coefficient of performance equal to 0.56. Thus the Melcor pump will draw 225 watts from its power supply to remove 125 watts of heat from saline water. Furthermore, the Sommers system (like the Aronson system) is physically large and bulky, thus being unsuitable for use as a home appliance for purifying small to moderate amounts (i.e., a few tens of gallons per day or less) of water.
Continuous freezing purification processes (in contrast to batch processes such as those of Aronson and Sommers) have also been employed for desalinization of a municipal water supply (for example with plant processing capability of one million gallons per day). Such processes have succeeded in desalinizing large volumes of water, but because they require complicated equipment with numerous moving components, have prohibitively high capital cost even for large-scale implementation.
It has not been known until the present invention how to freeze purify water using a simple, safe, economical, and reliable multiple chamber batch system, suitable for use as home or restaurant appliance. The inventive system has few or no moving parts, requires no scraping or transporting of the generated ice, requires no chemical or cartridge replacement, and can be embodied in a small housing suitable for use as a home or restaurant appliance to eliminate as much as 95 percent of the impurities in up to five gallons of water per day with electric energy consumption of about 16.7 BTU per pound of water processed (40 watt-hours per gallon processed).